This invention relates generally to disk drive systems. More particularly, this invention relates to a method and apparatus for determining an amount of power at which to write information to an optical disk.
Personal computers typically connect to an optical disk drive such as a CD-ROM to read data from a compact disk. On the compact disk, data is stored in the form of pits and lands patterned in a radial track. The track is formed in one spiral line extending from the inner radius of the disk to the outer edge. A pit is a location on the disk where data has been recorded by creating a depression in the surface of the disk with respect to the lands. The lands are the areas between the pits in the tangential direction. The reflectivity of the pits is less than the reflectivity of the lands. To store audio or digital information, the length of the pits and lands are controlled according to a predefined encoding format.
When reading information from the disc, light from a laser beam is directed onto the track and the light beam is reflected back to a photo-sensor. Since the pits and lands have different reflectivity, the amount of reflected light changes at the transitions between the pits and the lands. In other words, the encoded pattern of the pits and lands modulates the reflected light beam. The photo-sensor receives the reflected light beam, and outputs a modulated signal, typically referred to as an RF signal, that is proportional to the energy of the light in the reflected light beam.
In FIG. 1, relationship of the RF signal to the pits 26 and lands 28 is shown. A smaller pit 26 or land 28 decreases both the period and the amplitude of the RF signal. The RF signal in the pits 26 and lands 28 has opposite polarity.
One encoding format used in optical disk systems is eight-to-fourteen modulation (EFM). EFM reduces errors by minimizing the number of zero-to-one and one-to-zero transitions. In other words, small pits are avoided. A zero is indicated by no change in the energy of the reflected beam for at least two clock periods. A one is indicated by a change in the energy of the reflected light beam, that is, a pit edge. Applying the EFM encoding rules, a pit or land will have a length corresponding to an amount of time for at least three and up to eleven clock periods and the electronics will output a corresponding voltage as shown in FIG. 1.
When reading data, the RF signal needs to be decoded into a serial digital data signal. In one circuit, to decode the analog RF signal, a comparator compares the RF signal to a reference voltage to generate a digital data signal.
To write data to a CD-Recordable (CD-R) or a CD-Rewritable (CD-RW) disk, power is supplied to the laser which heats and melts a portion of the disk surface to create the pits. The optimum amount of power to supply to the laser depends on the characteristics of the disk, the optics, the laser, the temperature and the recording speed. The amount of write power is determined for each combination of recorder and recording speed at the time of recording.
When reading recorded data, the RF signal may be asymmetrical with respect to a predetermined reference voltage. For data written at different amounts of write power, when read, the amount of asymmetry in the corresponding RF signal varies in accordance with the amount of write power. To determine the optimum write power, random EFM data is recorded at different write powers. The recorded data is read back and the asymmetry of the data written at each write power is measured based on the peak voltage levels of the analog RF signal.
In FIG. 2, three RF signals 30-1, 30-2, 30-3 are shown. The data associated with each RF signal 30 was recorded at different write power levels. A line 36 representing a reference voltage is also shown. The highest peaks A1 represent the RF signal at a land, while the lowest peaks A2 represent the RF signal at a pit. Each waveform also shows the corresponding peak levels A1 and A2 for each waveform. Waveform one 30-1 was written at a write power much less than an optimum power level. Waveform two 30-2 was written at a write power equal to the optimum power level. Waveform three 30-3 was written at a write power much greater than the optimum power level.
Ideally, in a CD-R disk drive, the asymmetry is measured in terms of a parameter called Beta xcex2 which is the difference between the peak levels A1 and A2 of the analog RF signal normalized to the peak-to-peak value. In other words, Beta xcex2 is defined in accordance with relationship one as follows:                     β        =                              (                                          A                1                            +                              A                2                                      )                                (                                          A                1                            -                              A                2                                      )                                              (        1        )            
The optimum write power is associated with the value of Beta xcex2 closest to or equal to zero. A peak-bottom-hold circuit supplies digital values of the peak voltages, A1 and A2, to a processor which determines the value of Beta in accordance with relationship one, above.
In practice, in a CD-R disk drive, the value of Beta xcex2 is determined in accordance with relationship two as follows:                     β        =                              (                          P              +              B              -                              2                ⁢                                  V                  REF                                                      )                                (                          P              -              B                        )                                              (        2        )            
where P is the peak voltage, B is the bottom voltage. To provide a valid measure of Beta xcex2, the peak and bottom values should not have any DC bias. To measure the amount of DC bias in the peak and bottom signals, a low-pass filter filters the RF signal to provide a value VREF representing the DC bias. To remove the DC bias, in the numerator, VREF is subtracted twicexe2x80x94once from the peak value P and once from the bottom value B.
Ideally, in a CD-RW disk drive, a parameter called Gamma xcex3 is used to determine the optimum write power, rather than Beta xcex2. Gamma xcex3 is the normalized slope of the modulation amplitude m of the RF signal with respect to write power Pw. The modulation amplitude m of the RF signal is determined in accordance with relationship three as follows:                     m        =                                            I              11                                      I              top                                .                                    (        3        )            
I11 is equal to the peak-to-peak value of the RF signal at the lowest predetermined frequency. Itop is the envelope of the I11 xe2x80x9chighxe2x80x9d signal levels of the RF signal. The envelope is provided by a 100 Hz low-pass filter. The normalized slope of the modulation amplitude m with respect to the write power, that is, Gamma xcex3, is determined in accordance with relationship four as follows:                     γ        =                              (                                          ∂                m                                            ∂                WritePower                                      )                    ·                                    (                              WritePower                m                            )                        .                                              (        4        )            
In practice, the modulation amplitude m for data written at each write power is determined in accordance with relationship five as follows:                     m        =                                            (                              PeakAverage                -                Bottom                            )                                      (                              Peak                -                Bottom                            )                                .                                    (        5        )            
Peak is the largest peak value of the RF signal; Bottom is the lowest value of the RF signal; and, Peak Average is the average of the peak values for the data. The slope of the modulation amplitude m with respect to the write power, that is, Gamma xcex3, is determined in accordance with relationship six as follows:                     γ        =                                            (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  m                                m                            )                                                      (                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  WritePower                                )                            WritePower                                .                                    (        6        )            
Data is written consecutively at increasing levels of write power. The change in modulation amplitude (xcex94m) is the difference in modulation amplitude m between data written at consecutive write power levels. The change in Write Power (xcex94Write Power) is the difference between consecutive values of write power.
After determining Gamma xcex3 for a predetermined number of write power levels, the value of Gamma xe2x80x98xcex3xe2x80x99 that is closest to the value of a predefined Gamma-target xcex3target is identified. The value of the write power Ptarget associated with Gamma xe2x80x98xcex3xe2x80x99 is determined. The optimum write power PWO and erase power PEO are determined in accordance with relationships seven and eight, respectively, as follows:
PWO=xcfx81xc2x7Ptargetxe2x80x83xe2x80x83(7)
PEO=xcex5xc2x7PWOxe2x80x83xe2x80x83(8)
The symbol xcfx81 is a predefined multiplication factor to determine the optimum write power PWO, and xcex5 is the erase/write power ratio. The values of xcex3target, xcfx81 and xcex5 are predefined and recorded on the disk.
When data is read from the optical disk, that data may have bit errors despite determining the amount of write power based on the value of Beta xcex2 and Gamma xcex3, depending on the type of disk drive. The bit errors are detected by an error-detection circuit different from the circuits that provide the respective values of Beta xcex2 and Gamma xcex3. The error-detection circuit may check an error correction code embedded in the data for errors. The prior art methods using the values Beta xcex2 and Gamma xcex3, described above, do not consider the bit errors of the data when determining the write power.
In view of the foregoing, it would be desirable to provide a method and apparatus that selects a write power in accordance with the bit errors of the data.
A method determines a power value for writing data on a disk. Write-patterns are written on the disk, and each write-pattern is written at an associated power value. For each write-pattern of at least a subset of the write-patterns, the write-pattern is read, a bit error corresponding to the write-pattern is determined to generate a calibration profile of associated bit errors and power values, and a selected power value is designated based on the calibration profile.
In one embodiment, the selected power value is designated based on both the calibration profile and an asymmetry value associated with each write-pattern. In another embodiment, the selected power value is designated based on both the calibration profile and the slope of the modulation amplitude of the RF signal with respect to write power.
A circuit determines a power value for writing data on a disk. A write circuit writes a set of write-patterns on the disk at associated power values. An error detector determines the number of bit errors for at least a subset of the set of write-patterns. An executing module designates a selected power value based on the number of bit errors for each write-pattern of the subset of write-patterns.
By designating the write power based on the number of bit errors of the data, the invention more closely associates write power with the bit errors; therefore, the number of bit errors of the data may be reduced.